Certain vacuum systems include a fore pump and a diffusion pump. The fore pump evacuates a vacuum load chamber to a pressure on the order of 10.sup.-2 Torr. When such a pressure is reached, the diffusion pump is activated, to reduce the pressure in the vacuum chamber to a much lower pressure, such as 10.sup.-6 Torr.
Diffusion pumps are typically characterized by a pool of liquid that is vaporized by an electric heater. The vapor is directed by one or plural nozzles against condensation surfaces. The vapor is condensed on the condensation surface and returns, by gravity, to the pool. Typically, the condensation surface is cooled by a cooling fluid, typically water, in a cooling coil contacting the condensation surface.
If there is a failure in a cooling system including the cooling coil, typically manifested by a failure of the coil to contain coolant fluid, the condensation surface becomes excessively hot, whereby vapor incident on the surface does not condense. Thereby, the diffusion pump does not operate properly and the diffusion pump overheats. To prevent such overheating it has been generally the prior art practice to halt operation of the diffusion pump by turning off the heater for vaporizing the liquid in the pool.
Halting operation of the diffusion pump causes the vacuum load to increase in pressure to the vacuum which can be attained by the fore pump. When the problem associated with the cooling system has been rectified, it is necessary to restart the diffusion pump and to reduce the vacuum load pressure several orders of magnitude. This is a time consuming process which can have a deleterious effect on the operation of the equipment in the vacuum chamber.
It has been suggested that this time consuming operation can be obviated by controlling the rate at which the vapor is evaporated in response to an indication of the effectiveness of the fluid in cooling the condensation surface. The vapor is evaporated so that when the fluid is completely ineffective in cooling the surface, liquid is evaporated from the pool, but at a slower rate than when the fluid is effective in cooling the surface. The indication of the effectiveness of the fluid in cooling the condensation surface has been obtained with a single thermostat located along a temperature gradient between the pool and cooling coil. The thermostat has temperature hysteresis characteristics, whereby normally closed contacts of the thermostat open at a temperature associated with an excessively high pool temperature and reclose at a lower temperature. Opening and closing of the contacts respectively cause power to be removed from and applied to an electric heating coil for liquid, i.e. oil, in the pool. Thereby the pool oil is cyclicly heated and cooled with a net lower amount of energy being supplied to it than during normal operation. It has been found, however, that a single relatively inexpensive thermostat is not reliable because different thermostats have considerably different characteristics.
The temperature gradient is frequently established by a metal strap between the cooling coil and stainless steel well surrounding an electric heater coil in the center of the pool; this general configuration is disclosed in my U.S. Pat. No. 3,282,330. In my aforementioned patent the strap or bar is disclosed as being copper. For years, however, the strap of the commercial product made in accordance with the teachings of U.S. Pat. No. 3,282,330 has been made of brass because of the tendency for copper to oxidize and because copper has a very high thermal conductivity. The copper oxidation tendency should be avoided because it causes variations in the temperature response of the temperature sensor on the strap. The very high copper thermal conductivity should be avoided because of the high heat loss and resulting low thermal efficiency associated with it. A problem with the use of brass, however, is that after several years the strap breaks at the intersection thereof with the very high temperature well (normally about 500.degree.-550.degree. F.).
It is accordingly an object of the invention to provide a new and improved diffusion pump.
It is an additional object of the present invention to maintain a diffusion pump pumping capacity at a reduced level when there is a complete loss of pump coolant.
It is an additional object of the present invention to provide a new and improved method of and apparatus for enabling a diffusion pump to be restored to full operation rapidly in the event of a failure of a cooling system for a condensation surface thereof.
Another object of the invention is to provide a new and improved method of and apparatus for accurately determining the effectiveness of a fluid in cooling a diffusion pump condensation surface wherein inexpensive thermostats can be used.
A further object of the invention is to provide a new and improved diffusion pump configured to have a brass temperature gradient establishing element having a longer life than such elements on prior art pumps.